Japanese Patent Laying-Open No. 2010-187431 (Patent Document 1) discloses a three-phase four-wire uninterruptible power supply system supplying a three-phase AC voltage and a neutral-point voltage to a load. The uninterruptible power supply system includes a plurality of uninterruptible power supply devices connected in parallel with one another to a load. Each uninterruptible power supply device includes a converter converting a three-phase AC voltage from an AC power supply into a DC voltage, a capacitor smoothing the DC voltage, an arm generating a neutral-point voltage based on the DC voltage, and an inverter converting the DC voltage into a three-phase AC voltage. A battery supplying a DC voltage to the arm and the inverter during an outage is connected to the capacitor.
Japanese Patent Laying-Open No. 2011-142705 (Patent Document 2) discloses an uninterruptible power supply system including a plurality of uninterruptible power supply devices connected in parallel with one another to a load. The plurality of uninterruptible power supply devices share a battery. Each uninterruptible power supply device includes a converter, an inverter, and a bidirectional chopper. The converter converts AC power from an AC power supply into DC power. The inverter converts the DC power into AC power to supply the AC power to the load. During a normal condition in which the AC power supply supplies AC power, the bidirectional chopper stores a part of the DC power generated by the converter in the battery. During an outage in which the AC power supply stops supply of AC power, the bidirectional chopper supplies the DC power in the battery to the load.